In recent years, from the standpoint of the burden upon the environment and so on, attention is being directed to electric automobiles of types that perform driving and braking with an electric motor according to the amount that an accelerator pedal or a brake pedal is stepped upon. Here, since the electric motor is an electrical component, accordingly the responsiveness and the linearity of driving and of braking for such an electric automobile are outstandingly excellent, as compared to those of an automobile equipped with an internal combustion engine for performing driving and braking by using both of the engine and a hydraulic braking mechanism.
This is because the response speed of an electric motor is around ten times faster than that of a hydraulic mechanism, and is around a hundred times faster than that of an internal combustion engine. Moreover, the relationship between the actual torque value Tm generated by an electric motor and the value Im of the current to the motor (hereinafter also sometimes termed the “drive current value”) is given by the following Equation (1):Tm=Kt·Im  (1)
Here, the torque constant Kt may be obtained in advance by measurement. Note that, depending on the type of the motor, the torque constant Kt may be fixed or variable according to the motor current value Im or according to the rotational speed. Therefore, the actual torque value Tm is accurately ascertained by detecting the motor current value Im with a current sensor or the like during the operation of the electric motor. Moreover, the actual torque value Tm is easily controlled by controlling the motor current value Im. Due to this, techniques of various types have been proposed for an electric automobile in order to implement traction control whose levels of safety and of comfort are high as compared to that of an internal combustion engine or via control of brake hydraulic pressure.
As the first example of such a proposed technique, there is mentioned the technique for detecting the slip ratio λ and the friction coefficient μ during vehicle travel, and for controlling the range over which the driving torque of the electric motor increases and decreases on the basis of the slip ratio λ and the friction coefficient μ that have thus been detected (refer to Patent Document #1, hereinafter termed the “prior art 1”). With the technique of the prior art 1, the state of the road surface upon which the vehicle is traveling is ascertained by calculating the average value of the ratio of the friction coefficient μ to the slip ratio λ. And the increase or decrease of the driving torque is restricted, when the road surface is one upon which slippage can easily occur.
As the second example of the proposed technique, there is mentioned the technique to restrict the requested torque by performing (i) to obtain the slip ratio λ and the friction coefficient μ during traveling by calculation; and (ii) to calculate a maximal driving torque on the basis of the maximal friction coefficient as estimated from the slip ratios λ and the coefficients of friction μ thus calculated (refer to Patent Document #2, hereinafter termed the “prior art 2”). With the technique of the prior art 2, the maximal friction coefficient is estimated by selecting a μ-λ characteristic curve for the road surface upon which the vehicle is traveling on the basis of the correlation between the slip ratios λ and the friction coefficients μ that have been calculated up until the present time point.
As the third example of a proposed technique, there is mentioned the technique for restricting the driving torque on the basis of the permitted maximal torque derived by performing: (i) estimation of the slip ratio λ and the driving torque T during traveling; (ii) estimation of the friction coefficient μ on the basis of this slip ratio λ and this driving torque T that have thus been estimated; and (iii) derivation of a permitted maximal torque for this friction coefficient μ that has thus been estimated and for the current load in the vertical direction sequentially (refer to Patent Document #3, hereinafter termed as the “prior art 3”). With the technique of the prior art 3, the permitted maximal torque is obtained by estimating the friction coefficient μ by referring to the first table that gives the relationship between the slip ratio λ and the driving torque T, and the friction coefficient μ, and by also referring to the second table that gives the relationship between the friction coefficient μ and the permitted maximal torque for each value of the load in the vertical direction.